The present invention concerns an adhesive composition that can be used to bond textile fibers adhesively to a cross-linkable rubber composition. The invention also relates to textile fibers sized by means of the adhesive composition, a composite comprising the sized textile fibers and a cross-linked rubber composition that is permeable to light, and a tire body whose sidewalls comprise the composite.
The coloring of textile cables to make them visible through the sidewalls of a tire, in particular a bicycle tire, is well known. This expedient is adopted in order to enhance the appearance of the sidewalls by exposing to view the structure of textile cables provided radially inside a cross-linked rubber composition which is transparent or translucent, so that the cables can be seen through the rubber composition.
For example, British Patent GB-A-460 557 discloses a bicycle tire that comprises, a textile ply, over which a solution of liquid rubber containing a colorant is applied (column 1, lines 23-37) and, over this solution, a layer of transparent rubber is applied.
Also, International Patent application WO-A-88/3 483, discloses a bicycle tire whose sidewalls comprise a textile ply coated with a transparent elastomer. This textile ply displays a color shade which defines a background color for the sidewalls. In addition, decorative patterns whose color(s) differ(s) from that of the textile ply are incorporated in it.
To cause a textile fiber to adhere to a cross-linkable rubber composition, it is known that the fiber is first immersed in a sizing bath and the sized fiber is then dried and subjected to appropriate heat treatment. Before immersing in the sizing bath, the fiber is sometimes immersed in a preliminary bath based on epoxy resin in order to improve the quality of the subsequent bonding between the sized fiber and the rubber composition in the cross-linked state.
The sizing bath generally contains an aqueous thermosetting phenolic adhesive comprising at least one diene elastomer. For adhesive bonding of rubber compositions to textile cables, such as those usually used in tires, the adhesive employed most often is that known as xe2x80x9cRFL adhesivexe2x80x9d (resorcinol formaldehyde latex).
This RFL adhesive thus comprises an aqueous thermosetting phenolic resin and an elastomer matrix.
More precisely, the thermosetting phenolic resin comprises resorcinol (or a resorcinol/formol precondensate), formol and sodium hydroxide in the aqueous phase.
The elastomer matrix comprises one or more diene elastomers, such as natural rubber, which may be blended with a styrene/butadiene copolymer and/or a vinylpyridine/styrene/butadiene terpolymer.
Ammonia or sodium hydroxide are also added to obtain the RFL adhesive. In some cases the adhesive may also contain block isocyanates.
Experience has shown that the phenolic resin confers upon this adhesive a satisfactory adhesion between cables of textile fibers and the cross-linkable rubber composition.
However, a disadvantage of this RFL-type adhesive emerges when it has been used to attach cables of textile fibers to a cross-linked rubber composition that is permeable to light, such that the cables can be seen through the rubber composition.
In effect, RFL-type adhesives are particularly characterized by red or orange-red colors conferred upon them by the thermosetting phenolic resins. Such colors or color tints are not always desirable from an aesthetic standpoint for a favorable exposure to sight of the textile cables within the sidewalls.
Japanese patent application JP-A-60/199 982 presents a process for sizing and coloring textile cables, whose purpose is to confer upon the adhesively bonded cables light tints in place of the deep colors inherent in RFL adhesives, along with satisfactory adhesion to a rubber composition chosen to be essentially transparent so as to expose the colored cables to view.
A sizing solution is first prepared by mixing while stirring slowly:
100 parts by weight of an aqueous dispersion of latex and a condensation product of resorcin-formaldehyde (RFL), with
a solution containing 10 to 50 parts by weight of an ethylene-urea compound (more precisely, 4,4xe2x80x2-diphenylmethanebisethylene-urea),
and then adding to the mixture so obtained a non-aqueous dye in an amount ranging from 3 to 30 parts by weight, while stirring continues.
The textile cables are then immersed in the sizing and coloring solution, the sized cables are dried, and they then undergo a specific heat treatment at a temperature which depends on the quantity of the ethylene-urea compound used in the sizing and coloring solution.
The presence of the ethylene-urea compound in the solution is essential to confer a clear color upon the cables and satisfactory adhesion to the rubber composition bonded to the cables, since the presence of pure pigment alone in the solution does not enable this light color (i.e. a color other than the red, orange-red or brown-red of the RFL adhesive) to be obtained.
However, a major disadvantage of this sizing and coloring operation is the higher operating cost incurred by the use of the additional ethylene-urea compound in the sizing solution.
Japanese Patent application JP-A-60/139 876 also provides a process for sizing and coloring textile cables with a view to bonding them to an essentially transparent rubber composition. The process involves mixing a non-aqueous colorant with a sizing solution of the RFL type and using a sequenced polyisocyanate as an agent to promote adhesion to the rubber composition.
The purpose of the present invention is to overcome the above drawback concerning the use of an ethylene-urea compound to lighten the color of the RFL adhesive, without adverse effect upon the adhesion. The present invention is based on the surprising discovery that an aqueous pigment dispersion comprising at least one organic pigment can be used in an adhesive composition that can be used to bond textile fibers adhesively to a cross-linkable rubber composition, wherein the adhesive composition is the reaction product of the aqueous pigmentary dispersion and an aqueous thermosetting phenolic adhesive which comprises at least one diene elastomer. The resulting adhesive composition has a light color different from the red or orange-red colors inherent in thermosetting formophenolic resins such as RFL resin, while having no adverse effect on the adhesion between the textile fibers and the cross-linkable rubber composition.
The presence of the pigment in the form of an aqueous dispersion in the adhesive composition eliminates the need for an additional compound to obtain the light color desired, since the adhesive composition contains no ethylene-urea compound. Moreover, the aqueous pigmentary dispersion has no adverse effect on the adhesion of the thermosetting phenolic adhesive.